Motion capture animations to IK rig
Most of the retargeting tools copy the bone rotations from the .bvh motion capture into FK bones of the rig. After that the animation can be copied to IK bones through FK/IK snapping scripts. When doing this manually from .bvh to a simple IK rig, you will encounter 2 difficulties:

Problem 1: The IK rig has extra control bones that the .bvh does not have. The rig structures differ. You need to add these bones to the .bvh rig and drive them with constrains from the motion captured bones. You have to usually add IK Pole bones, Hip location,.. (depends on the complexity of IK rig what you need to add) so you have all the bone locations and rotations to drive all the control bones of the IK rig:



Here the yellow bone tracks head of a bone up the chain and tail of a bone down the chain with 50% influence to stay right in between them. The green bones are just parented to yellow ones.

Problem 2: Usually the source .bvh armature has different proportions than the rig we need to transfer the capture to. When copying from FK to FK, there are not so many issues with proportions, because only the rotations get copied. IK rig on the other hand also relies on position of its controls. So before copying those, the .bvh source rig's bones have to be edited to proportion.